Formation of Highly Absorptive Secondary Brown Carbon Through Nighttime Multiphase Chemistry of Biomass Burning Emissions

Abstract. Biomass burning is a major global source of both primary brown carbon (BrC) and reactive trace gases in the atmosphere, thus exerts significant impacts on global climate and regional atmospheric chemistry. However, a substantial gap remains in our understanding of the nighttime evolution of biomass burning emissions. Here we present prominent nighttime formation of secondary organic aerosol (Night-OA) with strong absorptivity but markedly different spectral dependence from that of primary biomass burning organic aerosols, which was observed during autumn of the Pearl River Delta region of China when biomass burning plumes prevailed. Our results demonstrate that the formation of Night-OA appeared high dependence on both magnitudes of afternoon biomass burning emissions and available oxidants of NO₂ and O₃. Active nighttime NO₃ radical chemistry was characterized by quick O₃ depletion and almost zero concentration of NO, and the rapid decrease of NO₂ coincident with the quick nitrate formation suggests that the rapid NO₂ consumption supplied the NO₃ and N₂O₅ reaction chains. However, the quickest Night-OA formation occurred when nitrate formation ceased and relative humidity reached maximum, and mainly added mass to aerosol water abundant diameter ranges. This co-variation demonstrates that gas-phase and aqueous-phase chemistry of biomass burning precursors likely coordinated to promote the quick nighttime formation of Night-OA. Findings of this study highlight the nighttime darkening of biomass burning plumes through multiphase reactions and the proposed secondary BrC formation mechanisms may have broad implications in climate and air quality effects of biomass burning, such as the interaction between biomass burning plumes with water abundant pyroconvection cloud.